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Also called minor planet. Astronomy. any of the thousands of small bodies of from 480 miles (775 km) to less than one mile (1.6 km) in diameter that revolve about the sun in orbits lying mostly between those of Mars and Jupiter.

Asteroid belt

The region of the solar system that is between the orbits of Mars and Jupiter and in which most asteroids orbit

Astronomical unit

A measurement equal to the distance between the Earth and the sun, a distance of about 150 million km or 93 million miles


Outer layer of a comet composed of escaping gas and dust producing a highly reflective halo


a small mass of dust and ice that orbits the sun in a long, oval-shaped path

Dwarf planet

in orbit around the sun, has sufficient mass for its self-gravity to form nearly round shape, has not cleared the neighborhood around its orbit and is not a satellite (Pluto, Eris, Ceres)


a former theory that the earth was at the center of the universe

Impact crater

a crater on the surface of a planet or moon produced by the impact of an asteroid, meteoroid, or comet

Inner planet

The planets closest to the sun; Mercury, Venus, Earth, and Mars

Jovian planets

the large, outer planets made of gas - Jupiter, Saturn, Uranus, & Neptune

Kuiper belt

the region of the solar system outside the orbit of Neptune that is occupied by small, icy, cometlike bodies


the luminous phenomenon observed when a meteoroid enters Earth's atmosphere and burns up, popularly called a shooting star


The part of a large meteoroid that survives its trip through the atmosphere and strikes Earth's surface


solid particles smaller than asteroids and comets that orbit the sun

Meteor shower

a brief period when the earth passes through a cluster of meteroids, causing more meteors than usual to be seen

Nebular hypothesis

a model for the origin of the solar system that assumes a rotating nebula of dust and gases that contracted to produce the Sun and planets

Oort cloud

(astronomy) a hypothetical huge collection of comets orbiting the sun far beyond the orbit of Pluto, A spherical region of comets that surrounds the solar system out to more than 1,000 times the distance from the sun

Outer planet

the 5 planets farthest from the sun - Jupiter, Saturn, Uranus, Neptune, and Pluto

Terrestrial planets

the name often given to the four inner planets: mercury, venus, earth, and Mars

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